1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a unique releasable latching structure for burnable absorber rods which readily permits attachment and detachment of such rods directly to and from the adapter plate of the fuel assembly top nozzle and facilitates consolidation of the spent absorber rods.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron abosrber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison rods at stationary positions in a nuclear reactor core are described in U.S. Pat. Nos. (3,361,857) to Rose and (3,510,388) to Wood.
Also, the availability of assemblies of burnable absorber rods on a rapid response basis is required at reactor fuel reload time. The present design of the burnable absorber assemblies, being similar to those illustrated and described in the first two patent applications cross-referenced above, includes a plurality of precisely spaced apart absorber rods and thimble plugs fastened at their upper ends to a support Plate which also mounts a central hold-down device. In view of the multiplicity of components which make up the absorber assemblies and the precise spacing required between them when they are assembled together, it has been found necessary to assemble the absorber assemblies at a manufacturing facility located remote from the reactor site. The final absorber assemblies are then shipped with the fuel assemblies to the reactor site. This means that the particular absorber assembly design must be specified well in advance of the time of actual reload.
However, it is desirable to have the flexibility of specifying the burnable absorber assembly configurations at the latest possible time so that the nuclear reload design can be fine tuned based on the latest reactor operations input. The ultimate absorber assembly configurations specified may advantageously include, for example, twelve burnable absorber rods and twelve thimble plugs per assembly or other combinations of absorber rods and thimble plugs. To accommodate these variations in configurations with the present design, within the time frame mentioned above, would require final assembly of the burnable absorber assemblies at the reactor site. This is not a practical alternative in view of the present design of the absorber assembly.
Consequently, a need exists for a different approach to absorber assembly design which will provide greater flexibility in arriving at what the final configuration should be, based on substantially current reactor operating information.